1. Field of the Invention
This invention relates to ionomers and more particularly it relates to ionomers having a low level of chain branching.
2. Description of the Prior Art
Copolymers of ethylene and acrylic or methacrylic acids partially neutralized with metal ions such as sodium or zinc ions (ethylene ionomers) are well-known and are tough molding resins with a combination of good tensile properties and excellent abrasion resistance. Their deficiency is that at low temperatures they become excessively stiff (rigid).
U.S. Pat. No. 3,264,272 discloses ethylene/methacrylic acid/acrylic ester copolymers and ionomers produced from these materials. Polymerization at high pressures of 50 to 3000 atmospheres and elevated temperatures of 150.degree. to 300.degree. C. in the presence of a free-radical polymerization initiator is disclosed.
German OLS No. 2341462 discloses copolymers and ionomers of (a) .alpha.-olefins of the formula R--C.dbd.CH.sub.2 where R is a radical selected from hydrogen and alkyl radicals of 1 to 8 carbon atoms; (b) .alpha.,.beta.-ethylenically unsaturated carboxylic acids of 3 to 8 carbon atoms, and (c) alkyl esters of .alpha.,.beta.-ethylenically unsaturated carboxylic acids of 3 to 8 carbon atoms where the aklyl radical has at least 4 carbon atoms. The reference discloses polymerization temperatures of from 150.degree. to 300.degree. C. The polymers obtained by the reference are disclosed to have improved oxygen permeability. The disclosure is very broad in that olefins containing up to 8 carbon atoms are encompassed, alkyl esters where the alkyl radical has at least 4 carbon atoms are encompassed and the reaction temperature is from 150.degree. to 300.degree. C. There is no disclosure of the importance of nor any requirements regarding short chain branching. There is no disclosure of the importance of copolymerizing low T.sub.g acrylates nor any relationship thereof with improved low temperature properties. The most preferred esters are disclosed to be butyl acrylate, isobutyl acrylate, butyl methacrylate, isobutyl methacrylate, tertiary butyl acrylate and tertiary butyl methacrylate. Of these, only butyl acrylate would give low T.sub.g polymers. The only Example in this reference relating to butyl acrylate was carried out at a polymerization temperature of 215.degree. C. and as such the polymer was not produced at sufficiently low temperature and thus would not have the required low level of polyethylene short chain branches. Accordingly, the product would not have the desired combination of excellent low temperature properties and high level of ethylene crystallinity.